1. Field of the Invention
The present invention relates generally to electronic sectionalizers and, more particularly, to an electronic sectionalizer having a conductor provided with an electronics package mounted on the external surface thereof.
2. Discussion of the Prior Art
High voltage power distribution systems are typically comprised of a main supply line that is connected to a number of branch or lateral lines. Normally, the main line is protected near its source of power by an automatic recloser or a reclosing circuit breaker which is operable to disable the entire system downstream of the recloser if currents above a certain magnitude are detected. Automatic reclosers and reclosing circuit breakers are particularly useful for enabling transient fault currents to clear after which time the recloser can again energize the circuit. However, if fault current conditions remain, the recloser after one or more attempts to re-energize the circuit will cease operation and cause the distribution system to remain in a deactivated state until attended by a repairman.
In the past, fuse links were often installed at the beginning of each lateral line to protect the line and isolate it from the rest of the distribution system where over-current conditions existed only in a particular lateral line. Many problems were observed, however, in attempting to coordinate the opening characteristics of the fuse links with the reclosing apparatus and in attempting to ensure that the fuse link would not melt and open the lateral line before the reclosing apparatus had an opportunity to deactivate the entire system. As a consequence, electronic sectionalizers have been developed which instead count the number of times that the recloser opens and closes the circuit. After a specified number of current flow periods or "shots", the electronic sectionalizer disables the lateral line during a subsequent dead interval when the reclosing apparatus has opened if over-current conditions in the lateral line protected by the sectionalizer are detected. For additional disclosure of electronic sectionalizers, reference is hereby made to U.S. Pat. No. 4,553,188 dated Nov. 12, 1985, and to U.S Pat. No. 4,768,010 dated Aug. 30, 1988.
In the aforementioned U.S. Pat. No. 4,553,188, an electronic sectionalizer is disclosed in which a metal casing provides a current path across a fuse mounting while the sectionalizer is in a conducting position. An electronic circuit inside the tube detects at least two successive current surges through the casing and, during the following dead period, actuates a chemical actuator to trip the sectionalizer out of its conducting position. By mounting the electronic circuit inside the casing, the circuit is protected against undesirable effects of electromagnetic interference produced by the high currents and high voltages associated with the sectionalizer.
However, although the internally mounted electronic circuit construction provides the advantage of protection to the circuit against electro-magnetic interference, this type of construction suffers numerous drawbacks. For example, by placing the electronic circuit inside the casing, it is exposed to the high temperatures present in the casing and will run hot and thus be susceptible to variations and premature aging.
In addition, because the hollow space within the casing is dedicated to the storage of the electronic circuit, it is not possible to mount other elements of the sectionalizer inside the casing. Such a consequence is not critical to the sectionalizer illustrated in the above-mentioned U.S. Pat. No. 4,553,188, since that sectionalizer may not be reset and does not require additional elements to carry out either the release or resetting of the casing. However, it would be desirable to provide a construction which permits all necessary components of the sectionalizer, as well as any other optional components to be provided, in as compact an assembly as possible.
Another drawback of employing an internally mounted electronic circuit is that the size of the circuit is limited to a size capable of being fitted into the internal space of the casing. Because of this limitation, the number of features capable of use in the construction is limited by the size of the casing. Other encumbrances of the internally mounted electronic circuit also exist. For example, the internally mounted electronic circuit precludes access to the circuit so that, once mounted in the casing, it becomes very difficult to perform any programming of the circuit. In addition, it is difficult to connect the circuit to the detecting circuit employed in the sectionalizer.